1. Field of the Invention
The present invention relates to lasing devices for use in high-speed fiber optical communication systems, and more specifically to Vertical Cavity Surface Emitting Lasers (VCSELs) with high modulation bandwidth. Moreover the present invention relates to optical interconnects including lasing devices with high modulation bandwidth. Finally, the present invention relates to a method for manufacturing high-speed lasing devices.
2. Related Art to the Invention
Lasing devices and in particular Vertical Cavity Surface Emitting Lasers commonly used in high-speed communication systems include a cavity sandwiched between two highly reflective mirrors or reflectors so as to form a resonator. The mirrors include several alternating layers of semiconductors of high and low refractive index and are doped with p-type and n-type dopants or impurities, respectively so as to form a p-n or a p-i-n diode junction. In a semiconductor laser the gain mechanism that generates the lasing is provided by light generation from the recombination of holes and electrons. The recombining holes and electrons are injected, respectively, from the p and n sides of the diode junction. In telecommunication applications, the recombination of carriers is generated by electrical pumping, i.e. by forward-biasing the diode junction. Commonly, the current in the lasing device is confined to an aperture of the laser by implanting ions into the lasing device structure everywhere except the aperture on the lasing device so as to increase the electrical resistivity of the material around the aperture. Alternatively, the current around the aperture of the lasing device can be inhibited by oxidizing the material around the aperture of the lasing device.
Semiconductor lasers are employed in telecommunication applications for building optical interconnects used in electronic devices. Such optical interconnects became in recent years widely used in electronic devices due their capability of supporting a much higher bandwidth than traditional cable interconnects. In this context, the development of optical modules for converting optical signals into electrical signals and vice versa plays a crucial role in a wide range of applications, such as mid-board applications using optical interconnects.
Semiconductor lasers, such as VCSEL, typically convey information according to two schemes. In the first scheme, the laser is maintained in a constant light-emitting state and the output intensity is modulated by means of an external modulator driven by an externally applied voltage. Since this first scheme requires a costly external apparatus, optical interconnects including VCSEL are generally directly modulated. Direct modulation involves changing the current input of the laser, or, in other words, modulating the current around the bias current so as to produce a time-dependent output in the optical intensity. Usually, the current is switched between two values, both larger than the threshold current of the device.
FIG. 7 shows a lasing device 4000 according to the state of the art. The lasing device includes a substrate 4030 made of semiconductor material, a first mirror 4300 and a second mirror 4100. The first and second mirrors 4300 and 4100 respectively include a stack of alternating semiconductor mirror layers 4310, 4320. The layers 4310 have a high refractive index while the layers 4320 have a low refractive index. The first mirror 4300 is doped with n-type dopants, while the second mirror 4100 is doped with p-type dopants. The lasing device 4000 further includes a cavity spacer 4200 between the first and second mirrors 4300, 4100. The cavity spacer 4200 includes a first cladding layer 4230 and second cladding layer 4210 and an active layer 4220. Finally, the lasing device 4000 includes a current-confining region 4020 which defines a current-confining aperture 4021. The current-confining region is formed in the second cladding layer 4210 immediately below the second mirror 4100.
FIG. 8 is a schematic drawing illustrating the working principle of the lasing device 4000 of FIG. 7. In particular, FIG. 8 shows the effect of direct modulation on the carrier density in the active layer of the lasing device 4000. The current is switched between two values, both larger than the threshold current of the device. In the lasing device 4000, the carrier density is not perfectly clamped, but swings with the injection current due to gain saturation with optical field intensity and gain reduction due to internal heating.
FIG. 8 shows the distribution of the intensity of the optical field along the active layer 4220. As can be seen from the dashed line plot, the intensity of the optical field is maximum in the zone of the active layer corresponding to the current-confining aperture 4021. The optical field intensity is generally lower at the periphery and higher in the center of the active region. Moreover, in the peripheral regions of the active layer, optical loss is also higher than in the center of the active layer. This leads to a lower stimulated recombination rate at the periphery of the active layer. Upon switching from the high current level to the lower current level, the carrier density in the active layer 4220 will also switch from a high to a lower level. As can be seen from FIG. 8, at a particular bias current above the threshold, the carrier density distribution in the active layer 4220 is illustrated by the dotted curve (3). Zone A of the active layer 4220 indicates the area where the optical gain reaches the threshold value. Outside zone A, the carrier density is not sufficient for the generated gain to reach the threshold value. In addition, within zone A, the carrier density is not constant, but is larger where the local temperature and/or local photon density are higher.
FIG. 8 illustrates the particular case, in which the carrier density increases towards the center of the aperture.
At a higher bias current, both the temperature and the photon density in the active layer 4220 increase. Consequently, the carrier density in zone A will also increase in order to maintain the gain at the threshold value. At a higher bias current, the carrier density in the areas surrounding zone A will also increase and will become high enough to generate a gain that reaches the threshold value in a zone B surrounding zone A. This behaviour is illustrated by the dashed curve (1). Consequently, the active region of the laser where the carriers and photons are strongly coupled through stimulated recombination will expand from zone A at a lower bias to zones A and B at a higher bias current.
Upon switching from a high bias to a low bias, the carrier density in zone A of the active layer decreases at a much faster rate than the density in zone B of the active layer 4220 due to a stronger stimulated recombination in the areas with a higher optical field intensity. Thus, the carrier density will have two peaks at the periphery of the active layer 4220 as shown in the solid curve (2). These excess carriers in zone B of the active layer 4220 will act as a reservoir from which carriers will flow from the periphery towards the center of the active layer 4220, thereby acting as a capacitance connected parallel to the active layer of the laser. This extends the fall-time of the lasing device 4000 and negatively affects its response to a modulating signal. Consequently, the design of common lasing devices limits the modulation bandwidth and the high speed performance of optical interconnects employing the lasing device.
More precisely, since during the high-to-low transition, the laser 4000 evolves towards a lower carrier density and a reduced stimulated recombination rate the laser 4000 slows down in adapting to the new, lower current level, thereby enhancing the effect of the excess carrier density at the periphery of the active layer on extending the fall time. Even if the optical field intensity returns to its nominal value after a current waveform is applied, the carrier density will not, thereby leading to a dynamical coupling from the past to the future causing, for instance, inter-symbol interference.
In addition, in devices wherein current confinement is obtained by using a layer of insulation oxide 4020, the effective parasitic capacitance associated with the isolation oxide defining the current confining aperture is determined by the capacitance across the oxidized layer 4020 in series with the capacitance of the diode junction underneath the oxidized layer. If the diode is unbiased, the effective capacitance is given by the oxide layer capacitance in series with the depletion capacitance of the diode, the latest being the lowest of the two. Under forward bias the capacitance of the diode will increase while its series resistance will decrease leading to an overall increase of the effective capacitance of the structure. The maximum capacitance is only limited by the oxide capacitance, which is relatively large.
Lateral carrier spreading out from the aperture formed in the oxidized layer 4020 can be significant and in a steady-state, this will lead to a leakage current. The lateral carrier spreading will also provide some degree of forward bias to the outer regions of the diode structure under the isolation oxide 4020, thereby leading to an increase of the effective parasitic capacitance of the device with bias.
The above described effect further limits the modulation speed of the lasing device and hence the modulation bandwidth.
In order to overcome the problems associated with direct modulation of common lasing devices, many solutions have been proposed for reducing the effect of carrier spreading out towards the periphery of the active layer. In particular, clamping of the carrier density inside the active region could be improved through reduced gain saturation and internal heating. Alternatively, proton implantation or patterned tunnel junction techniques may be used to additionally confine the carriers so as to reduce the carrier density at the periphery of the active layer.
The known techniques have, however, the disadvantage that the additional confinement is effective only if the lateral geometry of the carrier confinement features matches the transversal distribution of the optical field. Developing a device with the above mentioned design requires extensive design and is very complex and costly to realize.